A typical example of the system clock supplier incorporated in a microprocessor 1 is illustrated in FIG. 1. The system clock supplier is operable in two different modes, i.e., an internal oscillation mode of operation and a transmission mode of operation. The system clock supplier is accompanied by two input terminals 2 and 3 which are coupled to an external crystal resonator 4. The external crystal resonator 4 is replacable with a ceramic resonator. The system clock supplier comprises a first amplifier circuit 5, a resistor 6 and a second amplifier circuit 7 used for waveshape, and the first amplifier circuit 5 and the resistor 6 form in combination a feedback loop. When the system clock supplier operates in the internal oscillation mode, the feedback loop produces a raw system clock in association with the crystal resonator 4. The raw system clock is shaped by the second amplifier circuit 7 to produce a system clock which is supplied to a system clock line 8. On the other hand, when the system clock supplier is changed from the internal oscillation mode of operation to the transmission mode of operation, an external small clock is supplied to the input terminal 2 and, then, amplified and shaped by the first and second amplifier circuits 5 and 7 to produce a system clock. Thus, if the system clock supplier operates in the transmission mode, the input terminal 3 lies idle. In general, a packaging restricts the microprocessor to have a relatively small number of the terminals, and a new function usually demand additional terminals. Then, an idle terminal such as input terminal 3 is not desirable for the microprocessor.